An attenuator may adjust a magnitude of a signal by reducing the magnitude of the signal, and an analog-type attenuator, such as a voltage variable attenuator, may continuously attenuate a magnitude of a signal according to a value of an adjustment voltage. The attenuator is widely used in various electronic systems, and especially, in a case of a system which needs to very accurately adjust a magnitude of a signal, such as a phased array antenna, a radar system, or a multi-port amplifier, the attenuator needs to have excellent performance. These systems need to simultaneously adjust a phase, as well as a magnitude of a signal. When a variation in an output phase is large when an attenuation value of the attenuator is adjusted, a false operation may be caused in the system. The attenuator and other components, for example, a phase shifter, connected to the attenuator need to have a very excellent return loss characteristic in order to minimize mutual influence when a magnitude of a signal and a phase are adjusted.
FIGS. 1A to 1C are attenuator structures in the related art, and FIG. 1A is an attenuator 110 having a PI structure, FIG. 1B is an attenuator 120 having a T structure, and FIG. 1C is an attenuator 130 having a transmission or distributed structure. A varactor diode, a PIN diode, a FET element, and the like are variously used as a voltage variable resistor element used for manufacturing the attenuator, and MMIC attenuators using the aforementioned elements have been recently much manufactured according to development of a semiconductor technology. The attenuator having the PI or T structure of FIG. 1A or 1B is an example of a circuit using a FET element, and is easily manufactured with a simple structure and exhibits regular attenuation performance in a broadband, but attenuation performance, return loss, and insertion loss performance thereof deteriorate according to influence of parasitic components of the element as a used frequency band is increased. The transmission structure of FIG. 1C exhibits excellent performance in a relatively higher frequency band than the structures of FIGS. 1A and 1B, but it is difficult to achieve very excellent return loss performance.
A phase variation is generated in the aforementioned voltage variable attenuators of FIGS. 1A to 1C when an attenuation operation is performed, and especially, influence of the parasitic components are further increased in a high operation frequency, so that a phase variation is further increased. That is, in a system which needs to accurately adjust the magnitude of a signal, it is necessary to minimize the phase shift and the change in return loss during the operation of the attenuator. However, the aforementioned attenuator in the related art has a problem in that a relatively larger phase variation is generated, and the return loss is considerably changed according to attenuation. The variable attenuator in the related art is disclosed in Korean Patent Application Publication No. 2006-0044082, and the like.